Our invention relates to cathode ray tubes (CRTs) in general and, in particular, to a CRT of the type wherein two separate electron beams are derived from the beam emanated by a single electron gun for dual image display on a target screen. Our invention is directed more particularly to such a one-gun two-beam CRT having improved beam spot uniformity within a region of concern on the target screen.
U.S. Pat. No. 3,819,984 to Hawken, for example, discloses a dual gun CRT having a pair of electron guns together with a pair of deflection systems for individually deflecting the electron beams emitted by the guns, thereby providing dual light images such as signal waveforms on one and the same fluorescent screen. Although of immense utility for some purposes, this known CRT is objectionable because of its large bulk as an inevitable result of the use of two electron guns.
We have applied for a Japanese patent (laid open to public inspection as No. 53-17057) a one-gun two-beam CRT (substantially identical with the one shown in FIG. 1 of the drawings attached hereto) capable of dual image display from a single electron gun. We have later found, however, that our prior proposed one-gun two-beam CRT has some drawbacks. The first is that different focusing voltages act on the two beams depending upon whether they are deflected upwardly or downwardly. If each beam is properly focused on the upper part of the screen, it will fall out of focus on the lower part of the screen, and vice versa. We will discuss the reasons for this later on.
Another drawback of the prior one-gun two-beam CRT is that the two beams tend to focus differently on the screen unless the electron gun is assembled to a required degree of precision with which it is intended to operate. The known CRT has no means for compensating for such different focusing characteristics and so could not be manufactured in an easy, economical way.